Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
For example, in a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2004-327130, as shown in FIG. 33, a plurality of fuel cell stacks 2a are placed in a stack case 1a. Each of the fuel cell stacks 2a is formed by stacking a large number of cell units 3a at predetermined intervals.
In each cell unit 3a, an air electrode is exposed in the stack case 1a, and the air supplied into the stack case 1a through an air supply pipe 4a is supplied to the air electrode. The air after partially consumed is discharged to the outside of the stack case 1a through an air discharge port 5a. In the meanwhile, a fuel gas is supplied to each of the cell units 3a through an inlet port 6a, and then, the fuel gas is discharged to the outside of the stack case 1a through a discharge port 7a. 
As shown in FIG. 34, in a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2007-073357, a fuel cell stack 3b is provided in an apparatus body 2b covered by a heat insulating member 1b. The fuel cell stack 3b is formed by stacking power generation cells 4b and separators 5b alternately.
A first reformer 6b is provided along the fuel cell stack 3b in the stacking direction at a position for absorbing heat generated during power generation of the fuel cell stack 3b. Further, a second reformer 7b is provided at a position spaced away from the fuel cell stack 3b relative to the first reformer 6b, and a fuel gas flows into the second reformer 7b. The position of the second reformer 7b is determined such that the temperature of the fuel gas supplied into the second reformer 7b is raised to a temperature suitable for reforming reaction, using the exhaust gas from the fuel cell stack 3b as a heat source. The second reformer 7b is provided upstream of the first reformer 6b, and connected to the first reformer 6b in series.
In a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2007-080760, as shown in FIG. 35, a fuel cell stack 1c formed by stacking power generation cells and separators alternately is provided. The fuel cell stack 1c and a fuel reforming apparatus 2c are placed in a heat insulating housing 3c. The fuel reforming apparatus 2c includes a plurality of reformers 4c and a plurality of fuel heat exchangers 5c. An air heat exchanger 6c, a fuel gas supply pipe 7c, and an oxygen-containing gas supply pipe 8c are provided in the heat insulating housing 3c. 
In Japanese Laid-Open Patent Publication No. 2004-327130, the fuel cell stacks 2a each having a cylinder shape as a whole are placed in the stack case 1a, and the fuel cell stacks 2a are spaced considerably away from each other. In the structure, the overall size of the fuel cell system is considerably large.
Further, the sufficient air needs to be supplied to the air electrode of each cell unit 3a exposed in the stack case 1a. Therefore, the air pressure in the stack case 1a needs to be considerably high, and the pressure loss is increased.
Further, in Japanese Laid-Open Patent Publication No. 2007-073357, the fuel gas supply pipe 8b and the oxygen-containing gas supply pipe 9b are placed inside the heat insulating member 1b, in addition to the fuel cell stack 3b, the first reformer 6b, and the second reformer 7b. Thus, the overall size of the fuel cell system is considerably large.
In this regard, in the case where the space between the heat insulating member 1b and the fuel cell stack 3b is minimized to achieve reduction in the overall size of the fuel cell, due to the reforming reaction in the first reformer 6b and the second reformer 7b, i.e., due to the heat absorbing reaction, the temperature of the fuel cell stack 3b is decreased locally. Therefore, uniform temperature distribution in the fuel cell stack 3b cannot be achieved, and the power generation efficiency becomes low.
Further, in Japanese Laid-Open Patent Publication No. 2007-080760, the fuel cell stack 1c, a plurality of the reformers 4c, a plurality of the fuel heat exchangers 5c, the air heat exchanger 6c, the fuel gas supply pipe 7c, and the oxygen-containing gas supply pipe 8c are provided in the heat insulating housing 3c. Therefore, the overall size of the fuel cell becomes considerably large.
In this regard, in the case where the space between the heat insulating housing 3c and the fuel cell stack 1c is minimized to achieve reduction in the overall size of the fuel cell, due to the reforming reaction in a plurality of the reformers 4c, i.e., due to the heat absorbing reaction, the temperature of the fuel cell stack 1c is decreased locally. Therefore, uniform temperature distribution in the fuel cell stack 1c cannot be achieved, and the power generation efficiency becomes low.
Further, Japanese Laid-Open Patent Publication No. 2007-073357 and Japanese Laid-Open Patent Publication No. 2007-080760, seals for fuel gas and oxygen-containing gas manifolds are separated from each other in diagonal directions of the separator. In the structure, heat stress applied to the seals and the power generation units (cell units) becomes large.